1. Field of the Invention
The present invention relates to an optical recording medium driving apparatus that records and/or plays back a signal on an optical recording medium by using light radiation and a method of focusing light on a predetermined recording layer on the optical recording medium.
2. Description of the Related Art
Technologies for recording and playing back digital data include data recording technologies using optical disc recording media (including magneto-optical disks), such as compact discs (CDs), Minidiscs (MDs) (Registered Trademark of Sony Corporation), or digital versatile disks (DVDs). A laser beam is radiated on such an optical disc recording medium (also simply referred to as an optical disc) on which signals are recorded in pit or mark areas, and the signals are read out on the basis of the varied light beam reflected from the pit or mark areas.
Some optical disc recording media have multiple recording layers to increase the recording capacity. For example, DVDs having two recording layers are currently in widespread use.
In the case of optical disc recording media having multiple recording layers, a light beam is selectively focused on the individual recording layers to read out signals from the recording layers.
FIG. 12 illustrates how to focus light on such a multi-layer optical disc. An example of a focusing operation on the second recording layer on a two-layer optical disc is shown in FIG. 12. Of the two recording layers, the first recording layer is at the proximal side of an incident laser beam and the second recording layer is at the distal side thereof.
FIG. 12 schematically shows the focusing operation by using the waveforms of a light intensity signal (for example, a pull-in (PI) signal in the case of a four-divided detector) during focusing, a focus OK (FOK) signal, a focus error signal, and a focus drive signal and various threshold values.
In the focusing, an objective lens is driven toward the optical disc in response to the focus drive signal represented by a waveform to “ON POINT” in FIG. 12.
The light intensity signal generated when the objective lens is driven toward the optical disc is sliced with a predetermined threshold value th-FOK to generate the FOK signal, and an S-shaped curve of the focus error signal is detected during a period when the FOK signal is at a high (H) level. Specifically, an S-shaped curve of the focus error signal is detected under a condition in which the value of the focus error signal becomes lower than a threshold value th-2 after exceeding a threshold value th-1.
In the example shown in FIG. 12, since the laser light is focused on the distal second recording layer, the focusing is performed when the second S-shaped curve is detected. In other words, the focusing is performed when the value of the focus error signal becomes lower than the threshold value th-2 after exceeding the threshold value th-1 during a period when the FOK signal is at the H level again.
In recent years, high-density optical discs, such as Blu-ray discs (BDs) (Registered Trademark), have been developed, in addition to the CDs and DVDs, to further increase the recording capacity.
The BDs have disc structures including cover layers each having a thickness of about 0.1 mm. The BDs record and/or play back data under a condition in which both a laser having a wavelength of 405 nm (so-called blue laser) and an objective lens having a numerical aperture (NA) of 0.85 are used.
In the case of high-density discs, such as BDs, it is known that spherical aberration is caused due to a difference in the thickness between the cover layers above the recording layers. Particularly, since the cover layers of the different recording layers have different thicknesses on multi-layer optical discs, it is necessary to correct the spherical aberration.
When the correction of the spherical aberration is necessary, it is necessary to set a certain spherical aberration correction value in the focusing.
The spherical aberration correction value is set to a value appropriate for a target layer on which the focusing is performed in related art.
However, setting the spherical aberration correction value to a value appropriate for the target layer can prevent the S-shaped curves of the focus error signals on other recording layers from being appropriately detected. For example, when the focusing is performed on the target second recording layer, the sufficient amplitude of the focus error signal on the first recording layer can be prevented from being generated.
If the sufficient amplitude of the focus error signal on the first recording layer is not generated when the second recording layer is used as the target layer, it is not possible to appropriately focus the light on the target layer by the focusing method shown in FIG. 12. In other words, if the S-shaped curve of the focus error signal on the first recording layer is not detected, the S-shaped curve of the focus error signal on the second recording layer is erroneously recognized as the S-shaped curve of the focus error signal on the first recording layer. As a result, it is not possible to appropriately focus the light on the second recording layer.
For confirmation, when the focusing is performed on the target first recording layer, distortion of the focus error signal causes no problem. In other words, when light is focused on the first recording layer, it is sufficient to capture a light reflected from the recording layer for the first time when the objective lens is driven toward the optical disc. Accordingly, the amplitude of the focus error signal on the second recording layer is not allowed for. The focusing can be appropriately performed if only the sufficient amplitude of the focus error signal on the first recording layer is generated.
When it is necessary to correct the spherical aberration as in the case described above, the S-shaped curve of the focus error signal on the first recording layer may not be detected when the focusing is performed on the target second recording layer and the focusing may not be performed appropriately. In order to resolve these problems, for example, the focusing is currently performed by a method shown in FIG. 13.
FIG. 13 shows examples of the waveforms of a light intensity signal (PI signal), a FOK signal, a focus error signal, and a focus drive signal when the focusing is performed on the target second recording layer. In the example shown in FIG. 13, the first recording layer is represented as an “L1 layer” and the second recording layer is represented as an “L0 layer”.
In the example shown in FIG. 13, the spherical aberration correction value is set to a value appropriate for the second recording layer in the focusing on the second recording layer. Accordingly, the amplitude of the focus error signal on the first recording layer (L1 layer) is made smaller than that of the focus error signal on the second recording layer (L0 layer), thus causing distortion of the focus error signal.
In contrast, the light intensity signal has a sufficient amplitude even on the L1 layer. This shows little effect of the spherical aberration on the L1 layer.
Also in the method shown in FIG. 13, first, the objective lens is driven toward the optical disc, as represented by the focus drive signal.
In this example, two threshold values thP-H and thP-L are set for the light intensity signal. The FOK signal is generated so as to be at the H level when the value of the light intensity signal exceeds the threshold value thP-H and so as to be at a low (L) level when the value thereof becomes lower than the threshold value thP-L after exceeding the threshold value thP-H.
Then, time count is started when the FOK signal becomes at the L level and it is determined whether the S-shaped curve of the focus error signal is detected within a predetermined time X since the time count is started. The detection of the S-shaped curve of the focus error signal is performed under a condition in which the value of the focus error signal exceeds a threshold value thF-H shown in FIG. 13.
If the S-shaped curve of the focus error signal is detected within the predetermined time X, the time count is performed again and it is determined again whether the S-shaped curve of the focus error signal is detected within the predetermined time X. If the S-shaped curve of the focus error signal is not detected within the predetermined time X, the objective lens is driven in the reverse direction (in the direction away from the optical disc). After the first S-shaped curve of the focus error signal is detected, the focusing is performed. The first S-shaped curve of the focus error signal is detected under a condition in which the focus error signal exceeds a threshold value thF-L after the focus error signal becomes lower than a threshold value thF-ZL during a period when the FOK signal is at the H level.
Since the optical disc has only the two recording layers, it is determined that the S-shaped curve that is finally detected is the S-shaped curve of the second recording layer if no S-shaped curve of the focus error signal is detected again within the predetermined time X since the S-shaped curve of the focus error signal is detected. In this case, driving the objective lens in the reverse direction and performing the focusing in the first S-shaped curve allow appropriate focusing on the target second recording layer.
However, in the method shown in FIG. 13, it is necessary to reciprocate the objective lens in the direction away form the optical disc when the focusing on the second recording layer is performed. Accordingly, it takes longer time to perform the focusing in the method shown in FIG. 13, compared with the typical method in the related art shown in FIG. 12, in which it is sufficient to drive the objective lens only in one direction to perform the focusing on the second recording layer.
Technologies in the related art are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2006-155792 and Japanese Unexamined Patent Application Publication No. 2003-22545.